Cochlear electrode array with electrode contacts on medial side

ABSTRACT

An implantable electrode array, adapted for insertion into a cochlea, provides improved stability of electrode contact direction. In-line electrodes are spaced-apart along one side of a flexible carrier. The structure of the electrode array facilitates bending of the array with the electrode contacts on the inside of the bend, yet deters flexing or twisting of the array in other directions. The electrode contacts preferably are each made from two strips of metal, arranged in a &#34;T&#34; shape (top view). During assembly, all of the &#34;T&#34; strips are held in position on an iron sheet. Two wire bundles are formed that pass along each side of each &#34;T&#34;. The leg of each &#34;T&#34; is folded over to pinch at least one of the wires from one of the wire bundles therebetween. This pinched wire is then resistance welded to the strip. The sides of the &#34;T&#34; are then folded up and touch or nearly touch to form a &#34;Δ&#34; shape (side view). The wire bundles going to more distal electrodes pass through the &#34;Δ&#34; and are thus engaged by the &#34;Δ&#34;. A flexible carrier, made from, e.g., silicone rubber, is molded over and around the wire bundles and folded electrode T&#39;s, preferably in a slightly curved shape. The iron sheet is chemically etched away, leaving an array of spaced-apart electrode contact areas along one edge of the flexible carrier, each of which is electrically attached to at least one wire which passes through the carrier. The electrode array can be manufactured using low cost technology; and once made can be easily inserted, removed and reinserted, if required, into the cochlea or other curved body cavity.

This application is a continuation-in-part (CIP) of U.S. applicationSer. No. 09/140,034, filed Aug. 26, 1998, now U.S. Pat. No. 6,038,484and further claims the benefit of the following U.S. Provisional PatentApplications: Ser. No. 60/087,655, filed Jun. 2, 1998; and Ser. No.60/079,676, filed Mar. 27, 1998; all of which patent applications areincorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention relates to implantable stimulation devices, e.g.,cochlear prosthesis used to electrically stimulate the auditory nerve,and more particularly to an electrode array for use with a cochlearstimulator that is designed to place the electrode contacts of theelectrode array generally along one side of the array so that when thearray is implanted within the cochlea, or other body cavity, the side ofthe array whereon the electrode contacts are located can be positionedin close proximity to the cells that are to be stimulated, therebyallowing such cells to be stimulated with minimal power consumption. Forexample, where the array is implanted into the cochlea, the electrodeside of the array may be positioned closest to the modiolar wall,thereby placing all of the individual electrode contacts in closeproximity to the ganglion cells and thereby in close proximity to theauditory nerve fibers.

Hearing loss, which may be due to many different causes, is generally oftwo types: conductive and sensorineural. Of these, conductive hearingloss occurs where the normal mechanical pathways for sound to reach thehair cells in the cochlea are impeded, for example, by damage to theossicles. Conductive hearing loss may often be helped by use ofconventional hearing aids, which amplify sound so that acousticinformation does reach the cochlea and the hair cells. Some types ofconductive hearing loss are also amenable to alleviation by surgicalprocedures.

In many people who are profoundly deaf, however, the reason for theirdeafness is sensorineural hearing loss. This type of hearing loss is dueto the absence or the destruction of the hair cells in the cochlea whichare needed to transduce acoustic signals into auditory nerve impulses.These people are unable to derive any benefit from conventional hearingaid systems, no matter how loud the acoustic stimulus is made, becausetheir mechanisms for transducing sound energy into auditory nerveimpulses have been damaged. Thus, in the absence of properly functioninghair cells, there is no way auditory nerve impulses can be generateddirectly from sounds.

To overcome sensorineural deafness, there have been developed numerouscochlear implant systems--or cochlear prosthesis--which seek to bypassthe hair cells in the cochlea (the hair cells are located in thevicinity of the radially outer wall of the cochlea) by presentingelectrical stimulation to the auditory nerve fibers directly, leading tothe perception of sound in the brain and at least partial restoration ofhearing function. The common denominator in most of these cochlearprosthesis systems has been the implantation into the cochlea ofelectrodes which are responsive to a suitable external source ofelectrical stimuli and which are intended to transmit those stimuli tothe ganglion cells and thereby to the auditory nerve fibers.

A cochlear prosthesis operates by direct electrical stimulation of theauditory nerve cells, bypassing the defective cochlear hair cells thatnormally transduce acoustic energy into electrical activity in suchnerve cells. In addition to stimulating the nerve cells, the electroniccircuitry and the electrode array of the cochlear prosthesis performsthe function of the separating the acoustic signal into a number ofparallel channels of information, each representing the intensity of anarrow band of frequencies within the acoustic spectrum. Ideally, eachchannel of information would be conveyed selectively to the subset ofauditory nerve cells that normally transmitted information about thatfrequency band to the brain. Those nerve cells are arranged in anorderly tonotopic sequence, from high frequencies at the basal end ofthe cochlear spiral to progressively lower frequencies towards the apex.In practice, this goal tends to be difficult to realize because of theanatomy of the cochlea.

Over the past several years, a consensus has generally emerged that thescala tympani, one of the three parallel ducts that, in parallel, makeup the spiral-shaped cochlea, provides the best location forimplantation of an electrode array used with a cochlear prosthesis. Theelectrode array to be implanted in this site typically consists of athin, elongated, flexible carrier containing several longitudinallydisposed and separately connected stimulating electrode contacts,perhaps 6-30 in number. Such electrode array is pushed into the scalatympani duct to a depth of about 20-30 mm via a surgical opening made inthe round window at the basal end of the duct. During use, electricalcurrent is passed into the fluids and tissues immediately surroundingthe individual electrode contacts in order to create transient potentialgradients that, if sufficiently strong, cause the nearby auditory nervefibers to generate action potentials. The auditory nerve fibers arisefrom cell bodies located in the spiral ganglion, which lies in the bone,or modiolus, adjacent to the scala tympani on the inside wall of itsspiral course. Because the density of electrical current flowing throughvolume conductors such as tissues and fluids tends to be highest nearthe electrode contact that is the source of such current, stimulation atone contact site tends to activate selectively those spiral ganglioncells and their auditory nerve fibers that are closest to that contactsite. Thus, there is a need for the electrode contacts to be positionedas close to the ganglion cells as possible. This means, in practice,that the electrode array, after implant, should preferably hug themodiolar wall, and that the individual electrodes of the electrode arrayshould be positioned on or near that surface of the electrode arraywhich is closest to the modiolar wall.

In order to address the above need, it is known in the art to make anintracochlear electrode array that includes a spiral-shaped resilientcarrier which generally has a natural spiral shape so that it betterconforms to the shape of the scala tympani. See, e.g., U.S. Pat. No.4,819,647. The '647 U.S. patent is incorporated herein by reference.Unfortunately, while the electrode array with spiral-shaped carriershown in the '647 patent represents a significant advance in the art,there exists lack of sufficient shape memory associated with the carrierto allow it to return to its original curvature (once having beenstraightened for initial insertion) with sufficient hugging force toallow it to wrap snugly against the modiolus of the cochlea.

It is also known in the art, as shown in applicant's prior patents, U.S.Pat. Nos. 5,545,219 and 5,645,585, to construct an electrode carrierfrom two initially straight members, a rodlike electrode carrier and aflexible rodlike positioning member. As shown in these patents, the twomembers extend in substantially parallel relation to and closelyalongside each other, but are connected to each other only at theirrespective leading and trailing end regions. After implant, a pushingforce is applied to the positioning member so that it is forced toassume an outwardly arched configuration relative to the electrodecarrier, thereby forcing the electrode carrier into a close huggingengagement with the modiolus, thereby placing the electrode contacts ofthe electrodes in as close a juxtaposition to the cells of the spiralganglion as possible. The '219 and '585 U.S. patents are alsoincorporated herein by reference. The '219 patent, in particular,provides in FIGS. 1-10 and accompanying text an excellent summary ofprior art electrodes and the deficiencies associated therewith.

Unfortunately, while the electrode array taught in the above-referenced'219 and '585 patents has the right idea, i.e., to force the electrodecarrier into a close hugging engagement with the modiolus, it does soonly by use of an additional element that makes manufacture of the leadmore difficult and expensive, and only through application of anadditional pushing force which is applied to an electrode structureafter it is already fully inserted into the cochlea. Such additionalpushing force may easily cause damage to the delicate scala tympani.Moreover, the entire electrode array may twist during the insertionprocess, or when the additional pushing force is applied, therebycausing the electrode contacts to twist and/or be forced away from themodiolus, rather than in a hugging relationship therewith.

Thus, while it has long been known that an enhanced performance of acochlear implant can be achieved by proper placement of the electrodecontacts close to the modiolar wall of the cochlea, two main problemshave faced designers in attempting to achieve this goal. First, it isextremely difficult to assemble electrode contacts on the medial side ofthe an electrode array, facing the modiolus of the cochlea. Second,heretofore there has either been the need for application of an external(and perhaps unsafe) force, or a lack of sufficient shape memory, toallow the electrode (after initial straightening to facilitateinsertion) to assume or return to the desired curvature needed to placethe electrodes against the modiolar wall so that the curvature wrapssnugly around the modiolus of the cochlea. As a result, the electrodecontacts of the prior art electrodes are generally positioned too farway from the modiolar wall.

Many cochlear electrode arrays of the prior art are made for insertioninto a left cochlea, or a right cochlea, depending upon the orientationof the electrode contacts one to another. It would be desirable for auniversal electrode array to be made that could be used in eithercochlea, left or right, without concern for whether the electrodes wereorientated in a right or left side orientation.

It is thus evident that improvements are still needed in cochlearelectrodes, particularly to facilitate assembling an electrode so thatthe electrode contacts are on the medial side of the electrode array,and to better assure that the electrode assumes a close huggingrelationship with the modiolus once implantation of the electrode hasoccurred.

SUMMARY OF THE INVENTION

The present invention addresses the above and other needs by providing auniversal electrode array, adapted for insertion into either a left orright cochlea, which provides improved stability of electrode contactdirection. All of the electrode contacts are spaced apart along one edgeor side of the array, termed the "medial side". Advantageously, thestructure of the electrode array facilitates bending of the array withthe electrode contacts on the inside of the bend, yet deters flexing ortwisting of the array that would tend to position or point the electrodecontacts away from the inside of the bend. Hence, when inserted into thescala tympani duct of a cochlea, all of the electrode contacts on themedial side of the array generally face the modiolus wall of thecochlea.

In the preferred embodiment, the electrode contacts of the array eachcomprise two strips of metal, arranged in a "T" shape (as viewed from atop view of the strips). During assembly, all of the "T" strips are heldin a spaced apart, in-line, position on an iron sheet. Two wire bundlesare formed that pass along each side of each "T". The leg of each "T" isfolded over to pinch at least one of the wires from one of the wirebundles therebetween, which wire is then resistance welded to the strip.The sides of the "T" are then folded up and touch or nearly touch toform a "Δ" shape (as viewed from a side view of the strips). The wirebundles going to other electrodes of the array pass through the "Δ".Silicone rubber, or a similar substance, is molded over and around thewire bundles and folded electrode T's, to form the carrier. Preferably,the carrier is molded in a slightly curved shape in the region where theelectrode contacts are located. The iron sheet is chemically etchedaway, leaving an array of spaced-apart electrode contacts along one edgeof the flexible carrier, each having an exposed surface area that istypically flat with a rectangular shape. Each electrode contact area iselectrically attached to at least one of the wires which passes throughthe carrier.

Advantageously, the electrode array of the present invention can bemanufactured using easy, low cost technology; and once made can beeasily inserted, removed and reinserted, if required, into the cochleaor other curved body cavity.

In one embodiment, small non-conductive bumps or humps are formed in thecarrier between the electrode contact areas on the medial side of thearray. These small bumps are made, e.g., from a soft silicone rubber, orequivalent substance. When inserted into the cochlea, the small bumpsserve as non-irritating stand-offs, or spacers, that keep the electrodecontacts near the modiolus wall, but prevent the electrode contacts fromactually touching the modiolus wall. The bumps may also serve asdielectric insulators that help steer the stimulating electrical currentin the desired direction, towards the modiolus wall, as taught, e.g., incopending U.S. patent application Ser. No. 09/137,033, filed Aug. 28,1998, assigned to the same assignee as the present application, andincorporated herein by reference.

Once the electrode array of the present invention, with its electrodecontacts all facing the modiolus wall, has been inserted into thecochlea, a flexible positioner may be inserted behind the electrodearray so as to force the electrode contacts up against the modioluswall. The description and use of such a positioner is not the subject ofthe present application, but is described in Applicant's copendingpatent applications, Ser. No. 09/140,034, filed Aug. 26, 1998 now U.S.Pat. No. 6,038,484 and Ser. No. 09/216,063, filed Dec. 18, 1998, both ofwhich applications are assigned to the same assignee as is the presentapplication, and both of which are incorporated herein by reference.However, it is to be understood that although the positioner describedin the subject patent applications may be used with the electrode arrayof the present invention, the electrode array herein described is notlimited to use with such a positioner. Rather, because the electrodearray described herein will most often have its electrode contactsfacing in the medial direction without concern for twisting of thecarrier (and hence without concern for having the electrode contactspointing away from the medial direction), it offers advantages notheretofore available with prior art electrode arrays.

Insertion of the electrode array into the cochlea may be performed inconventional manner, e.g., using the electrode insertion tool describedin U.S. Pat. No. 5,443,493, incorporated herein by reference. Equivalentor similar insertion tools may also be used.

Advantageously, the electrode array of the present invention achievesthe following goals: (1) it helps assure that the electrode contacts ofthe electrode array will be optimally positioned facing the medialdirection, e.g., facing the modiolar wall in a cochlea of any size orany side (left or right) of the body; (2) it can be manufactured usingeasy, low cost technology; and (3) it can be easily inserted into thecochlea, and removed and reinserted, if required.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other features and advantages of the present inventionwill be more apparent from the following more particular descriptionthereof, presented in conjunction with the following drawings wherein:

FIG. 1 depicts an electrode array and associated lead for attachment toan implantable cochlear stimulator in accordance with the presentinvention;

FIG. 2 illustrates a side view of the proximal end of the lead of FIG.1;

FIG. 3 is a more detailed view of the offset portion of the lead/arrayof FIG. 1;

FIG. 3A is a sectional view taken along the line 3A--3A of FIG. 3;

FIG. 4 shows the electrode array of the present invention havingspaced-apart electrode array contacts along the medial side of thearray, which electrode array comprises the distal end of the lead/arrayof FIG. 1;

FIG. 5 shows a detail view of the electrode array contacts of theelectrode array of FIG. 4;

FIG. 5A is a sectional view of the electrode array taken along the line5A--5A of FIG. 5;

FIG. 6 shows an alternative embodiment of the electrode array of thepresent invention wherein bumps are formed in the space between eachelectrode contact;

FIG. 6A shows a detail view of the electrode array contacts of thealternative electrode array of FIG. 6;

FIG. 6B is a sectional view of the alternative electrode array takenalong the line 6B--6B of FIG. 6A;

FIG. 7A depicts a preferred manner of making a multi-electrode contactarray in accordance with the present invention;

FIG. 7B shows an enlarged view the "T" strips used in making theelectrode contacts of the array of FIG. 7A;

FIGS. 8A, 8B, 8C and 8D illustrate one manner in which wires are bondedand routed to each of the "T" strip electrode contacts of FIG. 7B duringmanufacture of the electrode array;

FIG. 9 depicts a molding die onto which the partially-formed electrodearray of FIG. 7A, with wires attached to each of the electrodes as shownin FIGS. 8A-8D, may be mounted in order to form a straight polymercarrier for the electrode array; and

FIGS. 10 and 11 illustrate a perspective and side exploded view,respectively, of an alternative type of molding die onto which thepartially-formed electrode array of FIG. 7A, with wires attached to eachof the electrodes as shown in FIGS. 8A-8D, may be mounted in order toform a curved polymer carrier for the electrode array.

Corresponding reference characters indicate corresponding componentsthroughout the several views of the drawings.

DETAILED DESCRIPTION OF THE INVENTION

The following description is of the best mode presently contemplated forcarrying out the invention. This description is not to be taken in alimiting sense, but is made merely for the purpose of describing thegeneral principles of the invention. The scope of the invention shouldbe determined with reference to the claims.

The invention described herein teaches a particular type of implantableelectrode array having multiple, in-line, electrode contacts. Here, theterm "in-line", used to describe the electrode contacts, means only thatthe electrode contacts are spaced apart more or less in alignment withthe longitudinal axis of a lead. It does not mean that a perfect,straight alignment with the lead axis must be achieved. For example,electrode contacts that zig-zag somewhat with respect to the lead axiswould still be considered to be in-line electrodes for purposes of thepresent invention. Thus, in general, "in-line" means that of twoadjacent electrode contacts, one will be more distal than the other.Further, all of the in-line electrode contacts will have an exposedsurface which, more or less, lies on the same side--the medial side--ofthe curved electrode.

The electrode array of the present invention may be best used with animplantable multichannel pulse generator, e.g., an implantable cochlearstimulator (ICS) of the type disclosed in U.S. Pat. No. 5,603,726,incorporated herein by reference, or other suitable stimulator. It is tobe understood, however, that although a cochlear electrode array ishereafter described, having dimensions suitable for insertion into thecochlea, the principles of the invention may be applied to other typesof implantable leads for applications other than cochlear stimulation.

The electrode array of the present invention is particularly adapted tobend or flex in one direction, thereby making it suitable for insertioninto a curved body cavity, such as the cochlea.

An important feature of the electrode array of the present invention isthat all of the active electrode contacts of the array are generallypositioned along one side, e.g., the medial side (the inside of thecurve or bend), of the array. Thus, when inserted into the curved orspiraling cochlea, which may advantageously be either a left or rightcochlea, wherein the cells to be stimulated are located within thecenter modiolus wall, the electrode contacts are positioned proximatethe modiolus wall, where they are closest to the cells to be stimulated.Hence, the electrode array of the present invention facilitatesstimulation of the desired cells at lower power levels than wouldotherwise be needed if the electrode contacts were not proximate themodiolus wall.

Another feature of the electrode array of the present invention is thatthe electrode contacts have, in the preferred embodiment, a relativelylarge exposed electrode surface area that is generally planar or flathaving a desired geometric shape, e.g., rectangular, semicircular, oroval. However, it is to be understood that the principles of theinvention may also be practiced with electrodes that have exposedsurface areas that are not flat, e.g., dimpled, or corrugated, orpitted, and that may have an exposed surface area that has irregulargeometric shapes.

Except as noted herein, the materials from which the electrode array ofthe invention is made, and the manner of making the electrode array, maybe conventional, as are known in the art.

A preferred electrode array 30 in accordance with the present inventionis shown in FIG. 1. The electrode array 30 forms the distal end of alead/array assembly 40 adapted to be connected to an implantablecochlear stimulator (ICS), not shown. The lead/array assembly 40includes the electrode array 30, a fantail proximal connector 42, and alead body 44 connecting the array 30 to the proximal connector 42. TheICS is typically housed within a ceramic or other case, such as isdisclosed in U.S. Pat. No. 4,991,152, incorporated herein by reference.The case has an array of feedthrough terminals corresponding to itsmultiple channels. A preferred ICS has eight channels, with each channelhaving two feedthrough terminals connected thereto. Such terminals aretypically labeled as M1 and L1 (for medial and lateral) for the firstchannel, M2 and L2 for the second channel, and so on, up to andincluding M8 and L8 for the eighth channel.

The feedthrough terminals are spaced across a header of the case. Insidethe case, each feedthrough terminal is connected to appropriateelectronic circuitry for the corresponding channel, as taught in thepreviously-referenced '726 patent. On the outside of the case, eachfeedthrough terminal is connected to a corresponding wire conductorwithin the lead/array assembly 40. Such wire conductors are identifiedin FIG. 1 by the numbers 1 through 16. The wire conductors 1-16 are ofnecessity spread out at the point where they connect to the feedthroughterminals of the header. Thus, the proximal end of the lead/assembly 40includes the fantail connector 42 that funnels the spread conductors1-16 at the point they connect to the feedthrough terminals down to thelead body 44. A side view of the fantail connector 42 is shown in FIG.2.

The manner of forming the fantail connector 42, and connecting it to thefeedthrough terminals may be conventual, and does not form part of thepresent invention. Rather, the present invention is directed to theelectrode array 30 at the distal end of the lead/assembly 40. It shouldbe emphasized that the electrode array 30 is not limited to use with aproximal fantail connector 42 and the type of ICS disclosed in the '726patent. Rather, the electrode array 30 may be used with any type ofproximal connector that interfaces with an appropriate pulse generator.

As seen in FIG. 1, the electrode array 30 is preferably curved anappropriate amount. A multiplicity of in-line electrode contacts 32 arespaced apart so as to lie on the medial side (inside of the curve) ofthe array. Sixteen such electrode contacts 32 are used in a preferredembodiment of the array 30. These electrode contacts are respectivelyconnected to the wire conductors 1-16 within the lead. As shown in FIG.1, the most distal electrode contact is connected to wire conductor 1within the lead 44, which in turn is connected to the feedthroughterminal L1 at the pulse generator. The second-most distal electrodecontact is connected to wire conductor 2 within the lead 44, and isconnected to the feedthrough terminal M1 at the pulse generator. In thismanner, the two-most distal electrode connectors 32 on the array may beconnected to the first channel of the implantable pulse generator. In asimilar manner, the two most proximal electrode contacts on the array 30are connected to wire conductors 15 and 16 within the lead 44, and areconnected to feedthrough terminals L8 and M8, corresponding to theeighth channel, of the implantable pulse generator. The other electrodecontacts 32 included within the array 30 are similarly connected to acorresponding channel within the pulse generator.

As further seen in FIG. 1, the preferred electrode array 30 alsoincludes three reference electrode contacts 34, identified in FIG. 1 bythe electrode numbers 17, 18 and 19. Such reference contacts 34 are notconnected to any wire conductors within the lead 44, and for this reasonare sometimes referred to as "dummy reference contacts". Rather, each ofthese reference contacts 34 may provide a reference indicator or markerto the physician inserting the electrode array relative to the depth ofinsertion.

As also seen in FIG. 1, the lead/array assembly 40 further includes anoffset portion 46 that effectively marks the end of the lead 44 and thebeginning of the electrode array 46. Such offset portion 46 facilitatesinsertion of the electrode array 30 into the scala tympani duct of thecochlea. The insertion process may be conventional, and is aided by aspecial tool of the type disclosed in the '493 patent, previouslyreferenced.

Turning next to FIG. 3, there is shown a more detailed view of theoffset portion 46 of the lead/array 40. A sectional view of the offsetportion 46, taken along the line 3A--3A of FIG. 3, is shown in FIG. 3A.As seen in these figures, the offset portion 46 separates the body ofthe lead 44 from the body of the array 30 by an offset distance L4. Whenmeasured from a center-line longitudinal axis 45 of the lead 44 to acenter-line longitudinal axis 35 of the array 30, this distance L4, inthe preferred embodiment, is about 1.3 mm. At the point of the offset,the diameter of the lead 44 is a distance L5, while the diameter of theelectrode array is a distance L6. In the preferred embodiment, both L5and L6 are about 0.8 mm. The length L9 of the offset portion 46 isapproximately 1.6 mm, allowing the wire conductors 1-16 within theelectrode array 30 to transition to the lead body 44 without too sharpof a bend. It is to be understood that these dimensions, as well asother dimensions presented herein, are only exemplary of one embodiment,and are not meant to be limiting.

Typically, as seen in FIG. 3, the body of the lead 44 may be made from asilicone rubber tube 43 that is inserted into the proximal end of theelectrode array 30 up to a specified distance L12 from the first activeelectrode contact 16. In the preferred embodiment, L12 is approximately3.0 mm, and the outer diameter of the tube 43 is approximately 0.64 mm.What this means, as a practical manner, as will become evident from thedescription below, is that the distal end of the tube 43 is positioned adistance L12 from the electrode contact 16 when the electrode array 30and offset portion 46 are formed through a molding process.

The material from which the lead/array 40, including the electrode array30, is made may be any suitable biocompatible material commonly usedwith implantable leads and other implantable components as is known inthe art. A suitable material, for example, is a type of silicone polymeror rubber known as LSR-70 or LSR-25. The properties of LSR-70 and LSR-25are well known in the art, and LSR-70 and LSR-25 may be obtainedcommercially from numerous sources, LSR-70 is formed into a desiredshape by injecting or otherwise inserting it into a mold while in aliquid state and allowing it to cure in the mold at a specifiedtemperature for a specified time period. For example, LSR-70 may cure ata temperature of 140 degrees C. for about 15 minutes. LSR-25 maylikewise be formed into a desired shape using a similar molding process,or it may be applied through a suitable applicator, e.g, a syringe, to adesired area and then formed into a desired shape. LSR-25 is essentiallythe same as LSR-70 except that when it cures it is significantly softer,i.e., more pliable. Both LSR-70 and LSR-25 readily adhere to the tubing43 so that when cured they become integral therewith.

Still with reference to FIG. 3, it is seen that the distance from theproximal end of the electrode array 30 to the proximal edge of electrodecontact 16 (i.e., the electrode contact 32 that is connected to wireconductor 16) is a distance L3. In the preferred embodiment, thedistance L3 is about 10.5 mm.

Next, with reference to FIG. 4, a more detailed view of the electrodearray 30 is shown. The electrode array includes electrode array contacts32 equally-spaced along a medial side of a flexible carrier 36. Theflexible carrier 36 is made from LSR-70, and is molded around anassembly of electrode contacts 32 and interconnecting wires as describedbelow in conjunction with FIGS. 7A-11. The electrode array 30 has anoverall length L7. Such length L7 is most easily measured when the array30 is straightened, as shown by the dotted lines in FIG. 3. In thepreferred embodiment, L7 has a value of approximately 25 mm. While theelectrode array 30 could be formed to assume any desired shape, in thepreferred embodiment it is formed to include a natural curve having aradius of curvature r2, with the electrode contacts 32 being positionedalong the inside of the curve. The radius of curative r2 may have avalue of approximately 9.0 mm.

As further seen in FIG. 4, a soft tip 37, having a depth of distance L8,is typically formed from LSR-25 at the very distal tip of the electrodearray 30. In the preferred embodiment, the tip 37 is made from amaterial that is softer than the flexible carrier 36, and the distanceL8 has a value of approximately 0.3 mm.

As additionally illustrated in FIG. 4, the reference marker contacts 34,identified as electrodes 17, 18 and 19, are spaced from the activeelectrode 16 a distance L11, with a spacing between the reference markerelectrodes of L10. In the preferred embodiment, the distance L11 isabout 3.0 mm, and the distance L10 is about 1.0 mm.

Referring next to FIG. 5, the preferred spacing between the individualelectrode contacts 32 is depicted. Such spacing, as well as all theother dimensional detail presented herein, is exemplary of a cochlearelectrode, and is not intended to be limiting. As seen in FIG. 5, eachexposed electrode contact surface area comprises a generallyrectangular-shaped area having a length L1 and a width W1. Other shapescould also be used. In the preferred embodiment, the rectangular area isroughly a square, with L1 and W1 each having a value of approximately0.4 mm±10%, thereby providing an exposed electrode surface area ofapproximately 0.16 mm². The spacing between corresponding points ofadjacent electrode contact areas 32 is a distance L2. L2 has a nominalvalue of approximately 0.9 mm±0.1 mm.

The electrode contact areas comprise an exposed surface of an electrodecontact 32 that is formed from folded strips 210 and 220 of abiocompatible metal, such as platinum, as described more fully below inconjunction with FIGS. 7A-8D. Such electrode contacts are embeddedwithin the molded carrier 36 as illustrated in the sectional view ofFIG. 5A, which is taken along the lines 5A--5A of FIG. 5. As seen inFIG. 5A, the carrier 36 is formed to have a cross-sectional area that isgenerally rectangular, having dimensions of X by Y mm, where the valuesof X and Y vary as a function of where along the length of the carrierthe cross section is viewed. At electrode 16 (near the proximal end ofthe electrode/array 30), for example, X and Y are both about 0.8 mm. Atelectrode 1 (near the distal tip of the electrode array), X and Y areboth about 0.6 mm. Thus, it is seen that the carrier 36 is tapered alongits length so that it has a smaller cross section at its distal tip thanit does at its proximal end.

Still with reference to the cross-sectional view of the array shown inFIG. 5A, it is seen that the sectional shape has rounded corners on theside opposite the medial side. (As explained previously, the medial sideis the side where the electrode contacts 32 are located.) The roundedcorners have a radius of curvature r1 that is approximately 0.3 mm inthe preferred embodiment.

The electrode contacts 32 have a general cross sectional shape, as seenin FIG. 5A, and as will be more evident from the description below ofFIGS. 7A-8D below, that resembles a triangle. The base of thistriangular-shaped (or "Δ-shaped") electrode forms the exposed electrodecontact area along the medial side of the electrode array, e.g., as seenin FIG. 5. The upward sloping legs 220 of this Δ-shape electrode extendinto the body of the carrier, e.g., as anchors, and thus become embedded(non-exposed) portions of the electrodes. It should be noted that whilein the preferred embodiment the upward sloping legs 220 touch at theirrespective tips to form the Δ shape, such touching is not required; noris the Δ shape required. What is important is that these legs 220 extendinto the body of the carrier, in some fashion, so that the electrode isfirmly anchored in its desired position along the length of the array.For example, in some embodiments, the legs 220 may be completely foldedover so as to lie almost flat on top of the exposed surface area, asshown generally in parent application (Ser. No. 09/140,034). In otherembodiments, the legs 220 may extend more or less straight into the bodyof the carrier, forming a generally block "U" cross-sectional shape.

Wire bundles 202 and 203 pass through the corners of the Δ-shaped (orother-shaped) electrodes and become embedded within the molded carrier36 when formed. As explained in more detail below, at least one wirefrom at least one of these wire bundles makes electrical contact witheach active electrode. The wires that do not make electrical contactwith an electrode contact are nonetheless engaged by or supported by theembedded portion of the electrode as they pass through the Δ (or other)shape. Such engagement helps support and position the wire bundles priorto molding the carrier over them. Moreover, the location of the wirebundles immediately behind and along opposing edges of the exposedsurface area of the electrodes helps add additional stiffness to theelectrode array, once formed, in the lateral direction, as explainedbelow, thereby making it more difficult to bend or twist the array inthe lateral direction. In contrast, the array remains relatively easy tobend in the medial direction. As used herein, the medial direction isthe direction of curvature defined by the radius r2 (FIGS. 4 and 6).

An alternative embodiment an electrode array 30' made in accordance withthe present invention is shown in FIGS. 6, 6A and 6B. This alternativeelectrode array 30' is the same as the array 30 illustrated in FIGS. 4,5 and 5A with the exception that a series of small non-conductive bumps,or humps 70, are formed between the electrode contact areas 32. As seenbest in FIG. 6B, these humps 70 have a height H1 of about 0.13 mm, andas seen best in FIG. 6A, have a width W2 of about 0.25 mm. As furtherseen best in FIG. 6, the humps 70 extend out from the medial surface ofthe electrode array. The humps 70 are made from a soft silicone rubber,or equivalent substance, such as LSR-25. When inserted into the cochlea,the small bumps 70 serve as non-irritating stand-offs, or spacers, thatallow the electrode contacts 32 to be positioned near the modiolus wall,but prevent the electrode contacts 32 from actually touching themodiolus wall. The humps 70 further serve as dielectric insulators thathelp steer the stimulating electrical current, flowing to or from theelectrode contacts, in the desired direction, from or towards the cellslocated in the modiolus wall, as taught, e.g., in the previouslyreferenced copending U.S. patent application Ser. No. 09/137,033. Exceptfor the presence of the humps 70, FIGS. 6, 6A and 6B correspond to FIGS.4, 5 and 5A.

One of the advantages of the present invention is that the electrodearray is easy and relatively inexpensive to manufacture. A preferredmethod of making the electrode array 30 or 30' is illustrated, forexample, in FIGS. 7A through 11. It is to be emphasized that the methoddepicted in these figures of making the electrode array is not the onlyway an electrode array 30 or 30' could be made. However, it representsan easy and inexpensive (and thus a preferred) way to make the electrodearray.

Most designs of electrodes and connectors are based on the principle ofmolding a contact or array of contacts, usually made from biocompatiblemetal, into a polymer carrier like silicone or polyurethane rubber. Theelectrode contacts are usually required to be located in a controlledposition in reference to the surface of the carrier, with specifiedsurface areas to be fully exposed to the stimulated or interconnectionarea. Disadvantageously, making such electrodes or connectors becomesextremely difficult, especially when the contacts are very small and/ora large number of contacts are required, e.g., as is the case with acochlea electrode. One of the main problems encountered in thefabrication of such electrodes or connectors is to find a reliablemethod of holding the system of contacts in the desired and stableposition during the process of welding the connecting wires and moldingthe polymer carrier. A further problem relates to maintaining acontrolled surface of the contacts that are to remain exposed, i.e., toensure that the contacts are not covered by the polymer when the carrieris molded.

The preferred methods of making the electrode array 30 or 30' describedbelow in connection with FIG. 7A through FIG. 11 are based on theprinciple of attaching (by the process of resistance welding) electrodecontacts made from precious, biocompatible material (such as platinum orits alloys) to a foil carrier made from a non-toxic butchemically-active metal, such as iron (Fe). Resistance weldingadvantageously provides a secure attachment of the electrode material tothe foil carrier without causing a deep fusion of the two materialsbeing attached. The resulting shallow fusion contact, in turn, allowsclean exposed electrode surface areas to be formed when the foil carrieris eventually chemically etched away, as explained below. Other types ofattachment that result in shallow fusion of the electrode material andthe foil carrier sheet material may also be used in lieu of resistancewelding.

Attached to the metal carrier, the electrode contacts remain in adesired and stable position allowing easy connecting of the wiringsystem and subsequent molding of the polymer carrier. After completionof the molding process, the metal foil carrier is chemically etched awayusing a mixture of diluted acids, such as HNO₃ and HCl. The preciousmetal contacts and polymer are immune to the acid and remain in theirintact, unaltered shape, and thereby provide the desired electrode arraystructure.

To illustrate the method, the method will be described relative to thefabrication of the electrode array 30 or 30' suitable for insertion intothe cochlea. As a first step, an array of contacts 200 are resistancewelded onto an iron carrier 100 so as to assume a desired in-linespaced-apart relationship, as shown in FIG. 7A. Each contact 200consists of two pieces of platinum foil 210 and 220, connected togetherand joined to the carrier 100 by a shallow-fusion spot weld 230, asshown in FIG. 7B. The width of the strip 210 is approximately W1, andthe width of the strip 220 is approximately L1. These strips arearranged to form a "T" shape, when viewed from a top view, with thestrip 210 forming the leg of the "T", and with the strip 220 forming thecross bar of the "T". Moreover, the legs of each "T", are arrangedin-line, with the proper spacing L2 therebetween, as shown in FIG. 7A.

As a second step, a wiring system is connected to each of the electrodecontacts 200. This is accomplished as shown in FIGS. 8A, 8B, 8C and 8D.As seen in FIG. 8B, for example, an insulated wire 202', is laid on topof the electrode foil piece 220 (the cross bar of the "T"). The leg ofthe "T" of the foil piece 210 is then folded over to hold the end of thewire while the wire is welded in position (FIG. 8B). The weldingprocess, preferably a resistance weld, burns away any insulation fromthe tip while making a secure mechanical and electrical connectionbetween the wire and the electrode contact 200. The result is anelectrode contact 200 having a wire 202' securely attached thereto (FIG.8C). If other wires are present, e.g., going to more distal electrodecontacts, then such wires may pass over the foil piece 210, lying moreor less parallel to the wire 202' so as to form a bundle of wires 202. Asimilar bundle may be formed on the other side of the folded foil piece210, thereby forming another wire bundle 203. The ends of the foil piece220 are then folded upwards to form, in a preferred embodiment, atriangle, or Δ shape (as seen in a side view), as shown in FIG. 8D.

As seen in FIG. 8A, at least one wire from one of the bundles 202 or 203is attached to the electrode contacts 2-16 in the manner describedabove. (For simplicity, only six of the sixteen or nineteen electrodecontacts used in the electrode array 30 or 30' are shown in FIG. 8A,)Typically, a wire from wire bundle 202 will connect to electrode contact16, and a wire from bundle 203 will connect to electrode contact 15, andso on, with adjacent in-line electrode contacts being connected to wiresfrom alternating wire bundles. At least two wires, one from each bundle202 and 203 remain for connection to the most distal electrodecontact 1. In this fashion, at least seventeen wires are used to makeelectrical connection with sixteen electrode contacts. In the preferredembodiment, for example, the wire bundle 202 may contain 9 wires, andthe wire bundle 203 may contain 8 wires, for the sixteen-electrode array30 or 30' described herein. The wire bundles 202 and 203 pass throughthe dummy electrode contacts, or reference marker contacts 34 (FIGS. 1,6), without making electrical contact therewith. For simplicity, thereference marker contacts 34 are not shown in FIG. 8A.

In one embodiment of the invention, an implantable electrode array isprovided having at least n electrodes, where n is an integer of at least8, and wherein the number of multiplicity of wires embedded within thecarrier of the electrode comprises at least n+1. In another embodiment,n may be an integer of at least 12, and the number of multiplicity ofwires embedded within the carrier of the electrode comprises at leastn+1. In a preferred embodiment, as described above, n is an integer ofat least sixteen, and the number of multiplicity of wires embeddedwithin the carrier of the electrode comprises at least n+1, or at leastseventeen.

Having a wire bundle on each lateral side of each electrode contact,e.g., as seen in the sectional view of FIG. 5A or 6B, and hence on eachlateral side of the electrode array, helps add lateral stability to thearray. This is true even when the wire "bundle" only contains one wire.Thus, an important feature associated with using two wire bundles in themanner described is that the wire bundles help add stiffness to theelectrode array in the lateral direction, but do not materially affectthe ability of the array to flex or bend in the medial direction.

Once the wire bundles 202 and 203 have been connected to all of theactive electrodes 200, the foil carrier 100 may be placed on a moldingdie 300 as shown in FIG. 9. The die 300 has alignment pegs 310 adaptedto align with corresponding alignment holes 110 in the foil carrier 100.The die 300 further has a cavity or channel 320 formed therein intowhich the required amount of material, e.g., LSR-70, needed to form thepolymer carrier 36 (FIGS. 4, 6) is injected. The LSR-70 is then cured inconventional manner. This cavity or channel 320 may be shaped or formedas desired. The mold depicted in FIG. 9 would form a straight carrier 36

As an alternative to the flat-surface die 300 shown in FIG. 9, a curveddie 301 is preferably used as shown in FIGS. 10 and 11. Such die 301includes a curved surface 303 on a holding block 304 on which the foilcarrier 100 may be placed. The block 304 has alignment pegs 311 adaptedto align with corresponding alignment holes 110 in the foil carrier 100.The foil carrier 100 is placed on the block 304 and bent over the curvedsurface 303. The die 301 is then placed over the block 304, with thefoil carrier 100 sandwiched therebetween. A channel or cavity 321 isformed in the die 301 having the desired shape and characteristics ofthe carrier that is to be formed through the molding process. Therequired amount of material to form the polymer carrier 36, e.g.,LSR-70, is then injected into the channel and allowed to cure. Byplacing the foil carrier assembly 100 in the curved die of FIGS. 10 and11 (note that FIG. 10 comprises a perspective view of the die 301 andblock 304, and FIG. 11 comprises a side or profile view of the die 301and block 304), the array can be molded or formed to assume the desiredcurved shape. Such curved shape is preferred to achieve directionalstability of the array during insertion.

Thus, it is seen that through proper use of the die 300 or 301/304, orother dies, the electrode array may be formed to assume a natural curvedshape, a slightly curved shape, or to be straight.

After the material used to form the carrier (e.g., LSR-70) cures, thefoil carrier with the electrode array assembly (which is now moldedinside of the polymer) is removed from the channel of the die 300 or301/304 and placed in a mixture of diluted acids. The mixture of dilutedacids dissolves the foil carrier 100, thereby exposing a clean surfaceof the electrode contacts 200. After washing to remove any residue ofacids and Fe salts, the main electrode array structure is completed.

Advantageously, the structure of the electrode array 30, as seen best inthe sectional view of FIG. 5A, or the electrode array 30', as seen bestin the sectional view of FIG. 6B, bends or flexes more easily in themedial direction than in the lateral direction. That is, the electrodearray, with its slight curved shaped, when inserted into the cochlea, isable to bend, as required, to follow the scala tympani duct of thecochlea (whether the right or left cochlea) as it is inserted deeper anddeeper into such duct. As it does so, the electrode contacts 32 remainclosest to and facing the modiolus wall, as desired. As the electrodearray is inserted deeper into the cochlea, the electrode array does noteasily twist, or bend laterally, which twisting or bending could movethe electrode contacts away from the modiolus wall. This is because theelectrode array is inherently stiffer in the lateral direction than inthe medial direction due primarily to the presence of the wire bundlesand folded/bent electrode contacts which provide an added degree ofstiffness in the lateral direction.

To further understand one mechanism by which the present inventionachieves flexing or bending in the medial direction, but resists suchbending in a lateral direction (where the "medial" direction may bedefined as the direction in which the electrode contacts face, and the"lateral direction" may be defined as a direction perpendicular to boththe medial direction and a longitudinal axis of the array), consider thefollowing simplified model of the electrode array: The electrodecontacts 32 may be viewed as rigid rectangular plates, hinged togetherby the flexible carrier material between each plate. Thus, sixteen suchplates are hinged together in a long chain, each plate in the chainbeing connected to an adjacent plate in the chain by way of a hingedconnection. Such chain of "hinged plates" may readily pivot about theirrespective hinged connections, thus easily and readily allowing thechain of hinged plates to bend in the medial direction. However, due tothe rigid nature of each plate, bending in the lateral direction,assuming a perfect hinged connection, is virtually impossible. Evenassuming a less-than-perfect hinged connection, bending in the lateraldirection is still made difficult. This is because fixed-length wirebundles are embedded in the carrier on opposite lateral sides of thearray. These "matched" (of equal length) wire bundles tend to makelateral bending or flexing more difficult because such lateral flexingor bending would typically require that one of the wire bundles increasein length, as the other decreases in length, as a lateral bend is made.

Because the electrode contacts of the electrode array disclosed hereinremain facing and closest to the modiolus wall, stimulation of the cellsembedded within the modiolus wall may occur at lower energy settingsthan would be required if the electrode contacts were not facing andclosest to the modiolus wall. Hence, use of the present electrode arrayallows desired stimulation to be achieved at lower power levels. Lowerpower levels, in turn, mean that the overall cochlear stimulation systemmay operate on less power, which usually means a longer interval betweenbattery replacement.

As described above, it is thus seen that the present invention providesan electrode array that is easy to manufacture and which providesenhanced performance when used. Such electrode array provides an arrayof spaced-apart electrodes along the medial side of the array. Uponinsertion into the cochlea, the electrode contacts all face the modioluswall. The composition and makeup of the electrode array makes it easierto bend in the medial direction than in a sideways or lateral direction.Thus, the electrode contacts remain on the medial side of the electrode,which medial side remains closest to the modiolus wall when theelectrode is inserted into the cochlea.

While the invention herein disclosed has been described by means ofspecific embodiments and applications thereof, numerous modificationsand variations could be made thereto by those skilled in the art withoutdeparting from the scope of the invention set forth in the claims.

What is claimed is:
 1. An implantable electrode array for use with atissue stimulation device comprisinga flexible carrier having a medialside; a multiplicity of in-line electrodes having an exposed surfacearea only on the medial side of the flexible carrier, the in-lineelectrodes being spaced along the flexible carrier from a distal end ofthe flexible carrier to a proximal end of the flexible carrier, thein-line electrodes each having an embedded portion behind the exposedsurface area that extends into the flexible carrier; and a multiplicityof wires embedded within the flexible carrier, wherein at least one wireof the multiplicity of wires is electrically and physically connected toa respective in-line electrode, and wherein each wire of themultiplicity of wires that is electrically connected to a more distalin-line electrode passes and is engaged by each more proximal in-lineelectrode; wherein the electrode array is more flexible in a medialdirection than in a direction lateral to the medial direction, whereinthe medial direction comprises the direction faced by the exposedsurface area of the in-line electrodes.
 2. The implantable electrodearray of claim 1 wherein each in-line electrode comprises first andsecond metallic strips formed in a "T" shape, wherein a leg of the "T"is folded and holds at least one of the multiplicity of wirestherebetween, the wire being electrically bonded to the folded T leg,and wherein sides of the "T" are folded upwardly into the flexiblecarrier.
 3. The implantable electrode array of claim 2 wherein themultiplicity of wires that are electrically connected to more distalin-line electrodes and engaged by more proximal in-line electrodes passadjacent to the folded up sides of the "T" of each more proximal in-lineelectrode.
 4. The implantable electrode array of claim 3 wherein thefolded up sides of the "T" form a "Δ" shape, the folded up sides of the"Δ" comprising the embedded portion of the electrode, and wherein themultiplicity of wires are grouped into first and second wire bundles,and wherein the first wire bundle passes adjacent to one side of the"Δ", and the second wire bundle passes adjacent to the other side of the"Δ".
 5. The implantable electrode array of claim 1 wherein themultiplicity of in-line electrodes comprises n electrodes, where n is aninteger of at least 8, and wherein a most distal electrode comprises afirst electrode, and wherein a most proximal electrode comprises an nthelectrode.
 6. The implantable electrode array of claim 5 wherein themultiplicity of wires comprises at least n+1 wires, at least one wirebeing connected to each of the second through nth electrodes, and atleast two wires being connected to the first electrode.
 7. Theimplantable electrode array of claim 6 wherein the at least n+1 wiresare separated into two wire bundles, a first wire bundle and a secondwire bundle, and wherein the first and second wire bundles are embeddedwithin the flexible carrier, the first wire bundle being routed up onelateral side of the flexible carrier and the second wire bundle beingrouted up the other lateral side of the flexible carrier, and wherein atleast two wires are connected to the first electrode, wherein one wireof the at least two wires connected to the first electrode comes fromthe first wire bundle, and wherein another wire of the at least twowires connected to the first electrode comes from the second wirebundle.
 8. The implantable electrode array of claim 1 further includinga tip at the distal end of the electrode array made from a material thatis softer than the flexible carrier.
 9. The implantable electrode arrayof claim 1 further including a hump formed on the medial side of theflexible carrier in the space between the exposed surface area of eachin-line electrode.
 10. The implantable electrode array of claim 9wherein the flexible carrier is made from a silicone rubber material ofa first hardness, and the humps are made from a silicone rubber materialof a second hardness, wherein the first hardness is harder than thesecond hardness.
 11. The implantable electrode array of claim 1 whereinthe electrode array comprises an implantable cochlear electrode arrayadapted for insertion into a cochlea of a patient, and wherein eachin-line electrode has an exposed contact surface area that isrectangular in shape.
 12. The implantable electrode array of claim 11wherein the exposed contact surface area of each in-line electrode issubstantially flat, having an area no smaller than approximately 0.16mm².
 13. An implantable cochlear electrode array for use with a cochlearstimulator comprisingan elongate flexible carrier having a medial sidefacing in a medial direction, wherein the elongate flexible carrier hasonly medial electrodes, and further wherein the flexible carrier has adistal end having a tip, wherein the tip is made from a material that issofter than the elongate flexible carrier; wherein the medial electrodescomprise at least eight spaced-apart electrodes embedded within theflexible carrier, each electrode having an exposed contact surface areaon the medial side of the flexible carrier; and a multiplicity of wiresembedded within the flexible carrier, at least one wire of themultiplicity of wires being electrically and physically connected to arespective electrode; wherein the electrode array is more flexible in amedial direction than in a lateral direction, wherein the medialdirection comprises a direction perpendicular to the medial side, andwhere the lateral direction comprises a direction perpendicular to themedial direction and the longitudinal axis.
 14. The implantable cochlearelectrode array as set forth in claim 13 further including a hump formedin the space between the exposed contact surface area of each electrode.15. The implantable cochlear electrode array as set forth in claim 13wherein the elongate flexible carrier has a natural curve in the medialdirection.
 16. The implantable cochlear electrode array as set forth inclaim 13 wherein the number of spaced apart electrodes comprises n,where n is an integer of at least twelve, and wherein the number ofmultiplicity of wires embedded within the carrier comprises at leastn+1, with at least two wires being connected to a most distal electrode,and with at least one wire being connected to the remaining electrodes.17. The implantable cochlear electrode array as set forth in claim 16wherein the at least n+1 wires are separated into a first wire bundleand a second wire bundle, wherein the first wire bundle is routed up onelateral side of the flexible carrier and the second wire bundle isrouted up the other lateral side of the flexible carrier.
 18. Animplantable cochlear electrode array for use with a cochlear stimulatorcomprisingan elongate flexible carrier having a medial side and alongitudinal axis; at least n spaced-apart electrodes embedded withinthe flexible carrier, where n is an integer of at least twelve, eachelectrode having an exposed contact surface area only on the medial sideof the flexible carrier; and a multiplicity of wires embedded within theflexible carrier, at least one wire of the multiplicity of wires beingelectrically and physically connected to a respective electrode, whereinthe number of multiplicity of wires comprises at least n+1, with atleast two wires being connected to a most distal electrode, and with atleast one wire being connected to the remaining electrodes; wherein theelectrode array is more flexible in a medial direction than in a lateraldirection, wherein the medial direction comprises a directionperpendicular to the medial side, and where the lateral directioncomprises a direction perpendicular to the medial direction and thelongitudinal axis.
 19. The implantable cochlear electrode array as setforth in claim 18 wherein the at least n+1 wires are separated into afirst wire bundle and a second wire bundle, wherein the first wirebundle is routed up one lateral side of the flexible carrier and thesecond wire bundle is routed up the other lateral side of the flexiblecarrier, and wherein both wire bundles are embedded within the flexiblecarrier.
 20. The implantable cochlear electrode array as set forth inclaim 18 further including a hump formed in the space between theexposed contact surface area of each electrode.
 21. The implantablecochlear electrode array as set forth in claim 20 further including atip at a distal end of the array made from a material that is softerthan the flexible carrier.
 22. The implantable cochlear electrode arrayas set forth in claim 20 wherein the elongate flexible carrier has anatural curve in the medial direction.
 23. An implantable electrodearray for use with a tissue stimulation device comprisinga flexiblecarrier having a medial side; a multiplicity of in-line electrodeshaving an exposed surface area on the medial side of the flexiblecarrier, the in-line electrodes having an embedded portion behind theexposed surface area that extends into the flexible carrier; and firstand second wire bundles embedded within the flexible carrier, each wirebundle comprising a multiplicity of wires bundled together at a proximalend of the flexible carrier, wherein the first and second wire bundlespass longitudinally through the flexible carrier near the embeddedportion of the in-line electrodes on opposite lateral sides of thein-line electrodes, wherein at least one wire of the multiplicity ofwires is electrically connected to a respective in-line electrode; andwherein the electrode array is more flexible in a medial direction thanin a direction lateral to the medial direction, wherein the medialdirection comprises the direction faced by the medial side of theflexible carrier.
 24. The implantable electrode array of claim 23further including a tip at the distal end of the flexible carrier madefrom a material that is softer than the flexible carrier.
 25. Theimplantable electrode array of claim 23 further including a hump formedon the medial side of the array in the space between the exposed surfacearea of each electrode.
 26. The implantable electrode array of claim 25wherein the flexible carrier is made from a silicone rubber material ofa first hardness, and each hump formed between the exposed surface areaof each electrode is made from a silicone rubber material of a secondhardness, wherein the first hardness is harder than the second hardness.27. The implantable electrode array of claim 26 wherein the electrodearray comprises an implantable cochlear electrode array adapted forinsertion into a cochlea of a patient, and wherein the exposed surfacearea of each electrode is rectangular in shape.